Inclination angles are conventionally sensed by measuring acceleration due to gravity using accelerometers mounted to a machine. When non-gravity induced accelerations are present during the machine's operation, the inclination sensor provides false readings. Therefore, there is a need to remove non-gravity acceleration errors.
Some examples of non-gravity induced accelerations are vibrations, changes in track/wheel speed, changes in machine speed causing wheel/track slip, and changes in pitch rate of the machine.
The non-gravity acceleration caused by wheel/track speed can be calculated by differentiating the wheel or track speed sensor signal. Conventionally, the inclination sensor is mounted to the machine's body so that it is aligned with the direction of travel (or at a fixed angle) and the calculated wheel/track acceleration can be directly subtracted from the inclination acceleration measurement. However, some machines have the inclination sensor mounted to a work tool that can be moved (e.g., bull dozer blade and or linkage). In this case, the calculated wheel/track acceleration and inclination sensor may not be aligned and the angle relative to the direction of travel changes.
Inclination angles are commonly determined by fusing inclination sensor data with integrated angular rate data using a Kalman filter. The integrated angular rate data is not affected by accelerations, but it contains a bias that drifts, causing errors. The Kalman filter uses the inclination sensor data to correct the integrated angular rate bias. The measurement variance of the inclination angle dictates how much the Kalman filter trusts the inclination angle measurement. When non-gravity accelerations occur, the false inclination causes the Kalman filter to incorrectly compensate the integrated angular rate bias. Methods to correct for the measurement error of the inclination sensor by removing as many of these non-gravity accelerations as possible may be desirable.
In other solutions, such as in U.S. Pat. No. 5,860,480, titled “Method and apparatus for determining pitch and ground speed of an earth moving machines,” higher accuracy angular rate sensors are used so that compensation to the inclination measurement is not as critical. Additionally, inclination sensors are typically mounted to the body of the machine and other sensors (e.g., cylinder position sensors, rotary position sensors), also present in the system of U.S. Pat. No. 5,860,480, are used to measure the work tool orientation relative to the body. In these situations the inclinometer is aligned with the direction of travel (or at a fixed angle) and the calculated wheel/track acceleration may be directly subtracted from the inclination acceleration measurement. Both options are expensive. Accordingly, there is a need for improved methods and systems for determining the relative angle between the machine's direction of travel and the orientation of an inclination sensor on a work tool.